In order to meet the requirement of a communication power supply for high power density and high efficiency, synchronous rectification technology is widely used in DC-DC power converters. However, since a rectifier diode will generate a considerable conduction loss, a rectification circuit composed of transistors replaces a circuit composed of rectifier diodes. Compared with the circuit using rectifier diodes, the circuit using transistors for rectification has lower circuit loss and high efficiency, and meanwhile has higher requirement for reliability of transistor control. Therefore it is needed to eliminate interference due to a driver, misoperation and other abnormalities, these abnormalities will cause conductivity of upper and lower switching tubes in the same bridge arm of a synchronous rectifier circuit, resulting in the damage to the switching tube. A synchronous rectification control method commonly used in the related art generally includes: detecting the output current in a secondary winding of the transformer unit, and generating a first driving signal according to the output current, and then driving the on-and-off of a rectifier in the synchronous rectifier circuit by using the first driving signal.
A current sampling device is used to detect the output current (i.e., the current flowing through the parasitic diode of the synchronous rectifier) in the secondary winding of the transformer unit and turns on the synchronous rectifier when detecting that there is a current flowing through. However, a current detecting circuit has parasitic parameters, the signal of the output current is easily delayed, easily causing delays of turn-on and turn-off signals. Meanwhile, when the first driving signal is converted from the signal of the output current, there also exists a delay and there are also interference signals because it is easily affected by a power circuit. When amplitudes of these interference signals exceed the base drive voltage threshold of the synchronous rectifier, the synchronous rectifier will be mis-opened, and further the synchronous rectifier is directly conducted to cause the damage to the synchronous rectifier.